The release of melanophore stimulating hormone (MSH) from the pars intermedia of the vertebrate pituitary is under an inhibitory control by an MSH release-inhibiting factor (MRIF). We propose to try to determine the comparative distribution, chemical structure, and mechanism of action of this brain hormone. Such work is presently in progress. We have clearly shown that tocinoic acid, the ring structure of oxytocin (L-Cys-L-Tyr-L-Ile-L-Gln-L-Asn- Cys-OH), possesses strong MRIF activity and that L-Pro-L-Leu-Gly-NH2, the side chain, (reported by others to be MRIF) is devoid of such activity. We plan to compare the relative potencies of tocinoic acid, its amide, tocinamide, and related partial structures of neurohypophyseal hormones (as possible candidates for the natural MRIF) on MSH release inhibition from the pituitaries of a number of vertebrate (amphibian, reptilian, mammalian) species. Analogs (e.g., deaminotocinoic acid and its amide, etc.) of these structures will be studied for their structure-activity relationships. These structures will be synthesized in our laboratory. It will be necessary to assess the relative MRIF activity of hypothalamic extracts of representative vertebrate species on both their own pituitaries as well as on that of other species to clarify the natural distribution and therefore probable similarity or differences in the structure of MRIF. We shall attempt to isolate MRIF. The ionic requirements for MSH release and for inhibition by the synthetic MRIF will be studied as will the possible role of an active transport (sodium pump) system in the mechanisms of release and inhibition. Some effort will be directed toward determining the optimal conditions (chemical constitution of the incubation medium, pH optimum, optimal incubation tme, etc.) for the study of MSH-release mechanisms.